Process for extraction of monomeric



Sept. 15, 1964 w BRAUN ETAL Re. 25,640

PROCESS FOR EXTRACTION OF MONOMERIC CAPROLACTAM FROM ITS POLYMEROriginal Filed Nov. 18, 1959 IN VEN TORS WIL HEl-M away y 4L TER ZEIINDE-E A T TORNEYS United States Patent 25,640 PROCESS FOR EXTRACTIONOF MONOMERIC CAPROLACTAM FROM ITS POLYMER Wilhelm Braun, Lucerne, andWalter Zehnder, Ems, Switzerland, assignors to Iuventa A.G. furForschung und Patentverwertung, Zurich, Switzerland Original No.3,047,565, dated July 31, 1962, Ser. No. 853,814, Nov. 18, 1959.Application for reissue Apr. 12, 1963, Ser. No. 274,377 Claims priority,application Switzerland Nov. 20, 1958 Claims. (Cl. 260239.3)

Matter enclosed in heavy brackets appears in the original patent butforms no part of this reissue specification; matter printed in italicsindicates the additions made by reissue.

The invention relates to a process for the production of lactams and,more particularly, to an increase in the quality of polycaprolactam byremoval of low-molecular constituents therefrom.

Upon refining and fabrication of e-caprolactam polymers, under certainconditions monomeric caprolactarn is present which, for seasons ofeconomics, should be reused as starting material for the polymerization.

Crude e-caprolactam polymers contain up to 12 percent low-molecularconstituents consisting of monomeric, dimeric and trimeric caprolactam.In order to obtain a spinnable polycaprolactam, it is necessary toremove the monomer from the crude polymerizate completely, if possible.Such purification can be accomplished, e.g., by extraction of thecomminuted material with water, vacuum evaporation of the low-boilingconstituents from the melt or evaporation of the monomers from the solidpolymer in vacuo or in an atmosphere of inert gas.

A suitable evaporation process has been described in Swiss Patent265,206, wherein solid crude polymer is heated in the absence of oxygenand, if desired, in the presence of an inert gas to ISO-200 C. wherebythe monomeric lactam distills.

Another process step whereby monomeric lactam is recovered as by-productis the after-condensation of polycaprolactam at elevated temperatures(e.g., according to US. Patent 2,557,808 or Belgian Patent 551,476). Inboth instances, at the refining of the crude polymer and at thepost-condensation, measures are taken to recover the monomeric lactam inpure and polymerizable form. When the polymer is treated in an inert gasatmosphere, a recovery and purification of the gas is provided in orderto recycle the same. The process suited for that purpose consists inconducting the gas stream containing the lowmolecular constituentsthrough or over water or other solvents for lactam whereby theconditions easily can be chosen in such a manner that the lactamentrained in the gas stream is absorbed by the solvent practically inits entirety. Although this process is simple enough, it suffers fromtechnological and economical drawbacks. For instance, on the one handrather dilute lactam solutions are obtained whose concentration requiresconsiderable energy. On the other hand, the inert gas, while being freedfrom lactam by the process, simultaneously is saturated with solvent inaccordance with the latters vapor pressure so that, before re-use asprotective or rinsing gas in the evaporation or post-condensation, itmust be subjected to a purification, i.e., an additional process step.Another disadvantage resides in the fact that vaporizable products whichmay be present in the polymer, e.g., plasticizers or lubricants, areobtained, during the treatment of the gas stream with solvents, in solidform and tend to clog the absorption system.

It now has been found that these drawbacks are averted when as anabsorbing agent from a gas stream for the low-molecular constituents ofthe poly-e-caprolactam, con- Re. 25,640 Reissued Sept. 15, 1964- sistingmainly of monomeric caprolactam volatile at elevated temperatures,liquid lactam is employed instead of water or other lactam-solvents. anabsorption temperature of approximately C. at which temperature thelactam vapor pressure is quite low, i.e., approximately 0.25 mm. Hg, sothat a solidification of the lactam need not be considered. Theevaporated lactam thereby is absorbed practically quantitatively, andthe circulating inert gas can be re-used without purification since itis not contaminated by solvents and contains very little monomericlactam due to the latters low vapor pressure. However, if the polymersubjected to the evaporation process contains comparatively much water,the water content of the inert gas rises because at the absorptiontemperature of over 70 C. water has so high a vapor pressure that it isabsorbed by the lactam only to a very slight extent. In that case it isadvisable to dry at least part of the inert gas before recycling intothe evaporator.

The monomeric caprolactam evaporated and absorbed in the liquid lactamin the manner described thereafter, naturally, assumes itself thefunction of an absorbent, i.e., it simply increases the quantity oflactam circulated and serving as absorbent. The excess lactamcorresponding to the absorbed amount is drawn ofi and can directly bereturned to the polymerization since it is pure and has a water contentof less than 0.1 percent even when the original crude polymer evaporatedhad a water content up to 2 percent beside the low-molecularconstituents.

The process according to the invention now will be further explainedwith reference to the accompanying drawing, a flow sheet, and thereafterin an example. However, it should be understood that both these aregiven merely by Way of illustration, not of limitation, and thatnumerous changes may be made in the details without departing from thespirit and the scope of the invention as hereinafter claimed.

In evaporator 1, crude polymer containing low-n1olecular constituentsand water is heated in a stream of inert gas. The latter is introducedthrough heater 11 and is conducted, together with evaporated monomericlactam, by way of line 2, into absorption column 3. Column 3 is loadedwith fillers to increase the contact surfaces. As absorbent andsimultaneously as a coolant, liquid lactam is injected into column 3through jet 5. This lactam, running downward over the fillers, is cooledby the counter-current stream of gas from which it absorbs monomericlactam.

In sump 6 the entire lactam, i.e., that introduced through jet 5 andthat absorbed from the gas, is collected and is reconducted to jet 5 byway of line 15, pump 7 and condenser 8. Simultaneously, continuously orin batches, the amount of lactam is drawn off from sump 6 throughoverflow pipe 16 provided with valve 14 which exceeds the constantquantity of lactam to be cycled. The lactam thus drawn oli correspondsto the amount absorbed in the inert gas stream. The latter,substantially free of lactam, leaves column 3 through line 9 and ispumped by pump 10 through heater 11 and back into evaporator 1.

For rinsing of line 12, continuously a small amount of fresh gas isintroduced, and a corresponding amount of circulating gas containingwater and very little lactam is drawn off through line 13. This rinsingcan be replaced, at least in part, in such a manner that at least partof the circulating gas before return into evaporator 1 after leaving theabsorption column is pumped by pump 17 through the drying device 18 andthere freed from Water.

EXAMPLE 1,000 kg. poly-e-caprolactarn shavings having an average size of2.5 mm., a monomeric lactam content of 10.5

Most advantageous is,

with lactam and is conducted through line 2 into the 2 absorption column3. This vertical column has an inside diameter of 60 cm. and, within alength of 400 cm., contains Raschig rings 4 of 15 mm. diameter asfillers.

7 m. /h. lactam are injected through jet 5 over the fillers, this lactamserving as lactam absorbent and coolant simultaneously. The risingnitrogen, substantially freed from lactam and cooled in the column to 76C., is recycled into the evaporator 1 by way of line 9, pump and heater11. The injected lactam, together with the monomer absorbed therein fromthe inert gas stream, is collected in sump 6 and is conducted, by meansof pump 7, to the Water condenser 8 where it is cooled to 75 C., andthence again to jet 5 Where it again is injected. 88 kg. pure lactam aredrawn oif through overflow pipe 16 during evaporation and absorption. m./h. fresh nitrogen are entered through line 12, and simultaneously anequal amount circulating gas is drawn 011? through line 13. From theevaporator, after termination of the evaporation, 907 kg.poly-e-caprolactam having an extract content of 1.2 percent are removed.

We claim as our invention:

1. A process for the recovery of volatile monomeric caprolactamentrained in a stream of inert gas, which comprises bringing said gasstream in contact with liquid monomeric caprolactam which acts assolvent for said 3 entrained caprolactam.

2. A process for the extraction of monomeric capro lactam'frompoly-e-capr olactam by evaporation, which comprises exposing saidpoly-e-caprolactam to an inert gas stream at a temperature ofapproximately C] C. thus washing volatile monomer out of saidpolye-caprolactam, and washing said gas stream containing volatilemonomer countercurrently with liquid monomeric caprolactam, at atemperature of approximately 75 C., thereby substantially freeing saidgas stream from entrained monomeric caprolactarn.

3. The process as claimed in claim 2, wherein said gas stream iscirculated continuously.

4. The process as claimed in claim 3, wherein part of said circulatedgas stream is withdrawn and replaced with fresh gas.

5. The process as claimed in claim 4, wherein said withdrawn gas isdried and again used as fresh gas.

6. A process for the extraction of monomeric caprolactam frompoly-e-caprolactam by evaporation, which comprises exposing saidpoly-s-caprolactam to a stream of nitrogen gas at a temperature ofapproximately [75 C;] 185 C. thus Washing volatile monomer out of saidpoly- E-caprolactam, and washing said nitrogen stream containingvolatile monomer countercurrently with liquid monomeric caprolactam, ata temperature 0 approximately 75 0, thereby substantially freeing saidgas from entrained monomeric caprolactam.

References Cited in the file of this patent or the original patentUNITED STATES PATENTS 2,867,805 Ludewig Jan. 6, 1959 0 2,891,633 Morroet al June 23, 1959 FOREIGN PATENTS 265,206 Switzerland Nov. 30, 1949

